1. Field of the Invention
Embodiments of the present invention generally relate to a method of sealing a cavity in a micro-electromechanical system (MEMS) or a nano-electromechanical system (NEMS).
2. Description of the Related Art
Many MEMS and NEMS devices require encapsulation in a low or very low pressure environment. This is especially true for transducers, such as those used in inertial sensors, which are affected by squeeze-film damping effects. In order to achieve this, methods such as Chemical Vapor Deposition (CVD) have been used to the seal cavities in which MEMS devices are enclosed.
These methods share a disadvantage, however, in that they use reactive gases in order to deposit the material needed to seal the cavity. These reactive gases can adversely affect the operation of the enclosed device during the sealing step but also after the cavity has been sealed. This phenomenon is particularly harmful to transducers, which may heat-up during operation and reach temperature levels which help to catalyze potential chemical reactions.
In order to solve this problem, methods for removing the residual gases were developed. A typical approach is to use a “getter” material. These materials, usually highly reactive metals, are capable of absorbing finite quantities of gas. This approach has several disadvantages, however, because the getter material will need to be inserted within the cavity housing the device. When wafer level packaging is used, the getter is usually deposited on the inner surface of a device package or on the inner surface of a wafer facing the cavity to vacuum seal. Wafer level packaging, however, is by definition not an integrated sealing method and can't be used to form small cavities. The getter material could also be deposited on the substrate, in the cavity containing the device. This methodology however would induce a dramatic increase in the cavity size meaning a lower density of device encapsulated per area. It would also complicate the existing process flows. Moreover, these disadvantages will be exacerbated by the nature of the getter material typically being incompatible with standard Complementary Metal-Oxide-Semiconductor (CMOS) processes, thereby requiring further alterations to the process flows. Another disadvantage with methods such as CVD is that material may be deposited inside the cavity, thereby interfering with the operation of the enclosed device.
Accordingly, there is a clear need for a method of sealing a cavity which is compatible with standard CMOS processes and which does not rely on getter materials or reactive gases.